Communications sometimes occurs across longer lines known as transmission lines. When the speed or frequency of signaling increases, shorter line lengths act as transmission lines. Special layout and termination of transmission lines is often needed to minimize reflections and optimize performance.
The shape of the waveform being driven onto the transmission line as an input can be adjusted to improve performance. The initial portion of the waveform can be increased in amplitude relative to the rest of the waveform, which is known as pre-emphasis.
FIG. 1A shows a prior-art FIR filter. An input signal, XN is altered or shaped by a finite impulse response (FIR) filter to generate output signal YN which is input to the transmit end of a transmission line. Delay elements 12 each delay the signal by 1/Z, where Z is a time-domain function and 1/Z is a one-unit delay. Delayed signals B0, B1, B2, . . . BN have increasing delays and are summed by summer 14 to generate output YN. Delay elements 12 may be flip-flops, latches or delay lines in an analog or a digital implementation of the FIR filter.
FIG. 1B is a graph of a FIR filter response. A bell-shaped curve has a maximum gain at a peak frequency with decreasing gain for frequencies above and below this roll-off frequency.
FIG. 2A shows a coded communication signal that is an input to a FIR filter.
FIG. 2B shows the coded communication signal after shaping by pre-emphasis using the FIR filter.
In FIG. 2A, a coded signal encodes the bit sequence 110010. A differential pair of lines carries the signal, with the solid line in the graph showing the true differential line and the dashed line in the graph showing the complement differential line. The coding of the bits stream onto the differential lines may be a non-return-to-zero (NRZ) code that switches differential lines on bit transitions.
When the bit stream changes from a 1 to a 0, or from a zero to a 1, the true and complement differential lines are switched. When the bit stream does not change, the differential lines do not switch. The bit stream may itself have been pre-encoded such as using a run-length limited code to ensure that bits switch frequently enough so that a clock may be extracted.
When a FIR filter such as shown in FIGS. 1A, 1B is applied to an input signal such as shown in FIG. 2A, the output waveform of FIG. 2B can be created. The output waveform of FIG. 2B contains pre-emphasis, since higher-amplitude spikes are added to each rising and falling edge of the waveform. These initial spikes may be 20 to 50% higher in amplitude than the flat portion of the waveform that follows each spike. These initial spikes can improve performance since a higher initial current occurs when first driving the transmission line during switching. Since the edge contains all the frequency components, and the following trace has a deterministic frequency response, pre-emphasis can be designed to cancel out the trace effect.
Waveform shaping may also be used on the far end of the transmission line, at the receiver. De-emphasis is sometimes used to shape the signal received from a transmission line. Higher frequency components of waveforms tend to attenuate more than lower frequency components over a transmission line. This frequency-dependent attenuation causes portions of the waveform after transitions to have a lower amplitude than portions farther away from transition edges at the far end of the transmission line. The graph of the differential signals, or eye pattern, closes immediately after the switching transitions. Pre-emphasis and de-emphasis both attempt to increase amplitude after transitions to open up the eye pattern.
FIG. 3 shows a prior-art receiver with a clock recovery circuit that controls a filter delay element. The input signal INPUT from the far end of the transmission line is applied to buffer 44 and to clock recovery circuit 48. Clock recovery circuit 48 may include a phase-locked loop (PLL) that locks to the highest frequency of transitions of input signal INPUT. Thus the bit clock is recovered from input signal INPUT by clock recovery circuit 48 to generate a bit clock.
The bit clock from clock recovery circuit 48 is applied to the clock input of latch or flip-flop 42, which delays the buffered input signal from buffer 44 by one bit-clock period. Summer 46 sums the buffered input signal from buffer 44 and the 1-bit-clock-delayed signal from flip-flop 42 to generate the de-emphasized output OUTPUT that can be used by the receiving system.
Receiver 40 uses the recovered bit clock to delay the input signal for filtering. Clock recovery circuit 48 may be difficult to design and operate, since transitions may not always occur at the maximum rate, such as when sequence of 2, 3, or more 1's occur.
FIG. 4A shows an ideal signal received by a receiver that is NRZ encoded. FIG. 4B shows the received signal after de-emphasis filtering. When the filter of FIG. 3 is applied to the ideal signal of FIG. 4A, the resulting waveform is shown in FIG. 4B and has been de-emphasized.
The delayed signal is inverted and added, or subtracted, from the buffered input signal, causing a larger signal to be produced for one bit-clock period after each switch transition of the input signal. After the first bit-clock period, the delayed signal changes to match the input signal, and the sum returns to the steady-state value. Thus a higher-amplitude initial pulse occurs for a pulse-width of exactly one bit-clock after each transition of the input signal.
While useful, the FIR filter is fairly complex and often has many delay stages. These delay states often require a clock. When a digital FIR filter is used, clocks are normally required, although an analog FIR filter may only need clocks for the delay stages. A simpler pre-emphasis device is desirable that does not require a clock. For de-emphasis, the delay flip-flop requires that the bit-clock be recovered from the input signal. Such clock recovery is undesirable. A de-emphasis system that does not need the recovered clock is desirable.
What is desired is a waveform shaping circuit that does not use a clock. A waveform shaping system that does not require a clock recovery circuit is desirable. Pre-emphasis and de-emphasis emulation without using a recovered clock is desirable.